Convert linux bootrom to external rom and fw_cfg

Convert linux bootrom to external rom and fw_cfg

We already have a working multiboot implementation that uses fw_cfg to get
its kernel module etc. data in int19 runtime now.

So what's missing is a working linux boot option rom. While at it I figured it
would be a good idea to take the opcode generator out of pc.c and instead use
a proper option rom, like we do with multiboot.

So here it is - an fw_cfg using option rom for -kernel with linux!

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>